The present invention relates generally to apparatus and methods of detecting the appearance and disappearance of wax particles in oil, and more particularly to determining appearance and disappearance temperature of wax particles in transparent, translucent and opaque oils.
Wax appearance temperature (WAT), also known as cloud point (CP), and wax disappearance temperature (WDT) are of fundamental and pragmatic significance to the petroleum industry. As defined by the American Society of Testing and Materials (ASTM) standard test methods D2500, D3117 or D5773, WAT or CP is the temperature at which haziness caused by formation of small wax crystals is first observed in a sample of crude or refined oil under prescribed cooling conditions. In an analogous manner, WDT is the temperature at which cloudiness caused by these wax crystals re-dissolve into liquid form under specified warming conditions.
The measurement of WAT and WDT is important for crude oil, since environmental changes may lead to phase transition and solids formation during oil production, oil storage in containers or tankers, or oil transport through pipeline, railway or trucks. The presence of wax crystals in the oil may restrict flow or plug a fuel filter. Depending on the rate of wax deposition or melting, WAT or WDT may define the lower limit of acceptable operability of equipment or processes associated with oil. For example, wax problems in production wells are very costly because of production down time for wax removal. Any deposition in a pipeline will cause a reduction in flow rates, and it is expensive and time-consuming to clean. As oil companies move into arctic environments and deep-water area for oil production in order to meet increasing oil demand, preventing and mitigating wax deposits becomes increasingly important.
The measurement of CP or WAT by ASTM method D2500 is limited to fuels that are transparent in a minimum of 40 mm thickness, and with a CP below 49° C. The method's sensitivity depends on the amount and size of wax, and the subjective judgment of the operator. It is a time consuming method. In contrast, most crude oils are opaque visually before reaching 40 mm in thickness and some may have WAT warmer than 49° C. Therefore D2500 is not applicable for use with certain oils.
Cross Polarized Microscopy (CPM) is one of the techniques for WAT measurement. To determine WAT, a sample is to be preheated and transferred to a microscope slide. Two polarizers are used: the first one restricts the light to undulate in only one direction, while the second one is positioned at 90° from the first polarizer and completely blocks the light wave. Wax crystals on the slide are detected by rotating the polarization plane of the linearly polarized light. The sensitivity of CPM depends on the size of wax and film thickness, as well as scale of magnification. However, the restricted field view makes it difficult to detect the first crystal. CPM requires some microscopic wax crystals to form for a detectable signal. The CPM method requires experienced operator to prepare the microscopy slides, set up the microscope and detect the first wax crystals from the images. It is therefore not a practical method to use in a daily routine as an analytical tool for WAT measurement.
Differential Scanning Calorimetry (DSC) detects WAT by measuring the difference in heat absorbed or released between a reference sample and the test sample at a given cooling or warming rate. The reference needs to have known properties and be thermally inert (i.e. does not form wax) over the temperature range of measurement. The WAT is detected by a deviation of experimental data from the reference baseline and typically requires a significant amount of wax formation for sensible detection. Consequently it may be difficult to obtain a reliable baseline and to pinpoint the deviation from the baseline, especially when crystallization rate is low and signal noise overshadows thermal effects. Measured WAT tends to be lower than the actual temperature of initial wax formation and interpretation of WAT depends on the experience of the operator.
Another method for measuring WAT is viscometry, which measures gradual change of rheological properties as wax precipitates. WAT is estimated by plotting viscosity versus 1/temperature. The sensitivity of this method depends on the amount and size of wax. It can detect WAT only when the volume fraction of crystals is large enough for the viscosity to increase exponentially to create a large enough change in signal for detection.
Filter plugging is another technique for measuring WAT. It is based on the continuous monitoring of pressure drop across a filter, while the sample flows through a temperature-controlled flow loop. To minimize the shear stress at the filter, 0.5 μm size filters are commonly used together with low flow rates. This is because at a high flow rate, the shear stress produced by the flow will tend to reduce the particle size and thus increase the amount of crystals required to plug the filter. The method depends highly on the flow rate used and the detection of WAT requires a significant amount of crystals with size larger than 0.5 μm to form. Filter Plugging is therefore more a metric of when sufficiently number of larger particles are formed to impede the flow rather than a measure of when the first wax crystal appears.
WAT is also measured by Fourier Transform Infrared Spectroscopy (FT-IR), which detects the increase in energy scattering due to wax solidification. The operator has to identify the linear regions in wavenumber and to calculate the WAT by determining the intersection of two nonparallel lines generated during when temperatures are higher and lower than the calculated WAT. This makes it difficult to detect the WAT if wax formation is gradual and the deviation from parallelism is subtle. Similar to DSC, this method requires large amount of wax for detection, and the interpretation of WAT depends on the operator's experience. Such a method is described in U.S. Pat. No. 6,841,779 B1 issued on Jan. 11, 2005 to Roehner et al.
Nuclear magnetic resonance (NMR) is also used for measuring WAT. The NMR parameters such as relaxation times are related to the chemical and physical properties of the sample. The method is found to be problematic and ineffective for crude oils with low wax. Such method is described in U.S. Pat. No. 7,688,071 B2 issued on Mar. 30, 2010 to Cheng et al.
Light scattering has been found to be a sensitive method for measuring WAT. A lens is used to concentrate light scattering caused by crystals and is extremely sensitive to small amounts of scattered light caused by tiny crystals. Such method is described in U.S. Pat. No. 5,088,833 issued on Feb. 18, 1992 to Tsang et al and implemented into ASTM D5773. It is applicable to testing relatively transparent samples but not opaque ones due to high opacity coefficient for visible light.
Due to limitations of various prior art methods, it is therefore beneficial to have a simple to practice, highly sensitive and precise method for measuring WAT/WDT of relatively opaque samples that is free of subjective operator interpretation.
It is an object of the present invention to provide apparatus and method of measuring WAT/WDT of relatively opaque samples of oil, while eliminating subjective operator interpretation.